The invention pertains to the field of bidirectional passband digital communication systems, and, more particularly to the field of improvements in head end or central office modems to remove the phase locked loops therefrom.
Digital data communication systems are well known in the art. Many treatises are available that describe them. Among these treatises are: Dixon, "Spread Spectrum Systems with Commercial Applications", Third Edition, 1994 (Wiley & Sons, New York) ISBN 0 471 59342-7; Stallings "Data and Computer Communications", 4th Ed. 1994 (Macmillan Publishing Co., New York) ISBN0-02-415441-5; Lee and Messerschmit, "Digital Communication, 2d Ed.", 1994 (Kluwer Academic Publishers, Boston), ISBN 0 7923 9391 0; Haykin, "Communication Systems" Third Edition 1994 (Wiley & Sons) ISBN 0 471 57176-8; Elliott, Handbook of Digital Signal Processing: Engineering Applications, (Academic Press, Inc. San Diego, 1987), ISBN 0-12-237075-9, all of which are hereby incorporated by reference. Generally, the problem which the invention is an attempt to solve is how to get rid of as many continuous tracking loops as possible in a bidirectional digital data communication system. The reasoning for this can be understood from the following discussion.
Digital data distributed communication systems can be baseband systems or passband systems. In baseband systems, the transmission media has the capability of transmitting digital pulses between widely separated transmitter and receiver locations. Passband systems require that the digital data be modulated onto a carrier frequency for transmission over the media.
Receivers for digital data passband systems can be either coherent or noncoherent. In coherent systems, the receiver has a local oscillator, usually taking the form of a phase locked loop (PLL) which is part of a continuous tracking loop and is maintained in constant phase lock with the phase and frequency of the carrier on which the received data is modulated. Coherent systems can make use of modulation schemes which alter either the phase, frequency or amplitude or any combination thereof of the carrier in accordance with the information content of the digital data to be transmitted. Incoherent systems do not have the local oscillator at the receiver phase locked to the carrier phase and frequency. In these systems, the designers have chosen to ignore the phase of the received signal at the expense of some degradation of the system performance and throughput.
Coherent systems can utilize binary or M-ary amplitude shift keying (ASK), phase shift keying (PSK) or frequency shift keying (FSK), as well as M-ary amplitude phase keying (APK) of which QAM (quadrature amplitude modulation) is a special case. Incoherent systems are limited to binary or M-ary ASK, FSK or differential phase shift keying (DPSK).
Coherent systems are higher performance systems because they have an additional degree of freedom for use in the modulation scheme which means more complex constellations of symbol sets can be used and more data bits can be encoded in each symbol in the constellation. This translates to greater throughput.
However, coherent systems are more complex since they require additional tracking loop circuitry at the receiver to recover the transmitted carrier and use the information so derived to steer the local oscillator so as to maintain its phase and frequency locked to the phase and frequency of the carrier. Usually the local oscillator being steered in the receiver is a PLL or has a voltage controlled oscillator negative feedback system in it (which is at the heart of almost every tracking loop). Carrier synchronization has been achieved by any one of a number of different ways in the prior art including use of PLLs where the carrier is not suppressed or Mth power tracking loops or Costas tracking Loops where the carrier is suppressed. Mth power and Costas tracking loops also contain voltage controlled oscillators as part of the tracking loop. The problem is that PLLs and negative feedback voltage controlled oscillators in tracking loops can and often do lose lock especially where there is rapid change in phase or frequency caused by conditions in the transmission media. When a PLL or other tracking loop loses lock, the system goes out of synchronization and fails to communicate data--its sole purpose in life.
All digital data communication also requires clock synchronization in the receiver to the clock in the transmitter because data is sent during discrete times. These discrete times are variously called chip times, bit times or symbol times in the prior art references. The importance of synchronization of the clock in the receiver to the clock in the transmitter is that in all forms of modulation, the amplitude, phase or frequency of the carrier (or some combination of these) must be sampled during each chip time so as to determine which symbol in the alphabet or code set in use was transmitted during that chip time based upon the phase, amplitude or frequency characteristics of the carrier during that chip time.
Receiver clock synchronization can be done on either a long term basis or a short term basis. Short term clock synchronization is called, amazingly enough, asynchronous transmission, but in fact the receiver clock is periodically synchronized to the transmitter clock at the beginning of transmission of each "character". A character is a collection of 5 to 8 symbols which are transmitted over a very short time (usually the symbols or binary bits that only have two states). The receiver clock resynchronizes during each character at the beginning thereof and need not resynchronize until the next character starts. Asynchronous transmission is cheap and less complex since timing synchronization problems caused by transmission of long uninterrupted streams of bits is avoided by sending the bits one character at a time and requiring synchronization between the receiver clock and transmitter clock only during that character.
The problem with asynchronous transmission is the high overhead. Each character of 5 to 8 bits must include a start bit, 1 or 2 stop bits and a parity bit. The start bit is used by the receiver to resynchronize its clock. The overhead of 2-3 bits per character of 5-8 bits makes asynchronous transmission inefficient to transmit large volumes of data. Asynchronous transmission can be extended to sending several characters grouped together with a preamble which is long enough for the receiver to synchronize to transmitted before every group of characters and a tracking loop to maintain the receiver clock in synchronization with the transmitter clock during the transmission of the group of characters. The concepts of the invention are applicable to asynchronous transmission where there is a tracking loop in the remote unit receiver but no tracking loop in the central unit receiver and only a periodic or occasional phase adjustment of the master clock and master carrier phase for use by the central unit receiver.
Synchronous transmission is a more efficient way of transmitting data since blocks of symbols or bits can be transmitted without start and stop codes. Sampling by the receiver during the middle of each bit or chip time is accomplished by keeping the receiver clock in synchronization with the transmitter clock. This maintenance of clock synchronization has been done in the prior art in many different ways. For example, a separate clock line can connect the transmitter and receiver, but this is impractical in many situations. A way of avoiding this is to embed the clock information in the data signal transmitted from the transmitter and recover the clock in the receiver.
Clock recovery has been done in a number of different ways in the prior art including transmitting the clock along with the data bearing signal in multiplexed form and using appropriate filtering of the modulated waveform to extract the clock. Another method is to use a noncoherent detector to first extract the clock and then processing the noncoherent detector output to recover the carrier. Where clock recovery follows carrier recovery, the clock is recovered from demodulated baseband signals. The early-late gate symbol synchronizer has also been used in the prior art to synchronize the receiver clock to the transmitter clock. This type clock recovery takes advantage of the fact that a matched filter output of a filter matched to a rectangular clock pulse is a triangular waveform which can be sampled early before the peak and late after the peak. By changing the timing of the sampling until the early and late samples have equal amplitude, the peak of the matched filter output signal can be found, and this will have a fixed phase relationship to the clock phase. This information is then used to steer a voltage controlled oscillator in a negative feedback system. Again, complicated circuitry centered around a voltage controlled oscillator is needed to recover the clock.
A technique called remote loopback or remote clock has been used in the prior art on, for example T1 type digital data communication phone lines. This technique is similar to the aspect of the invention involving having the remote unit local clock synchronized to the central unit master clock and using that local clock at the remote unit receiver for the remote unit transmitter. It is also similar to the aspect of the invention of using the central unit master clock, after adjustment in phase to synchronize it to the phase of the received clock from the remote unit transmitter, as the clock signal from the central unit receiver.
Since PLLs and tracking loops are not always reliable, and add complication and expense to receivers, it is desirable to be able to get rid of them wherever possible. Thus, a need has arisen for a bidirectional digital communication system where continuous tracking loops in the central unit receiver (or the receiver in the unit having the transmitter which transmits with the master clock and master carrier signals) have been eliminated.